When driven by a signal at a substantially constant reference frequency, the output signal of an electrothermal filter will experience a certain phase-shift.
WO/2006/132531 describes an oscillator based on thermal diffusion, which comprises a frequency locked loop which is coupled to an electrothermal filter and allows measurement of the absolute temperature T of the electrothermal filter. The frequency of the oscillator is inversely proportional to 1/T1.8. Using this oscillator it is observed that the phase shift between the driving signal of the electrothermal filter and the phase of the periodic signal at the output of the filter is a nearly linear function of temperature.
To avoid significant self-heating in the substrate in which the electrothermal filter is realized, the filter's power dissipation must be minimized. As a result, the filter's output will be small and will be obscured by noise. There is therefore a need for a phase detection circuit that can output a signal which is an accurate measure of the phase difference between a signal obscured by noise and a reference signal with the same frequency.
U.S. Pat. No. 4,520,320 describes a phase detection circuit for determining the phase difference between an input signal obscured by noise and a reference signal, where both signals are at the same frequency. This circuit consists of a chopper (a polarity reversing switch) embedded in a feedback loop that also consists of an integrator and a voltage-controlled phase shifter. The input signal is applied to the input of the chopper, while the control input of the chopper is coupled to the output of a voltage-controlled phase shifter. The input of the phase-shifter is a reference signal at the same frequency as the input signal. The output of the chopper is applied to an integrator which provides the control voltage to the voltage-controlled phase-shifter. The feedback loop will settle when the average input of the integrator is zero, which corresponds to a 90° phase difference between the input signal and the output of the phase-shifter. A further output of the voltage-controlled phase shifter is arranged to be 90° out of phase with the output supplied to the chopper and is thus in phase with the input signal obscured by noise.
The phase detection circuit of the prior art does not provide an output signal that is an accurate measure of the phase difference between the input and reference signals. Although the control voltage applied to the voltage-controlled phase shifter is related to this phase difference, their relationship is ill-defined, since the phase-shifter is a complex analog system that is sensitive to the tolerances and linearity of its constituent parts.
Another drawback of phase detection circuit of the prior art is that it is sensitive to any offset present at the output of the chopper. This offset may be caused by the asymmetric switching spikes produced by a practical chopper.
A further source of error arises if the duty-cycle of the signal applied to the control input of the chopper is not exactly 50%. In a practical implementation, this will occur if the rise and fall times of this signal are mismatched. The phase detection circuit will then be sensitive to even harmonics of the reference frequency present in the input signal. In particular, it will be sensitive to the DC level of the input signal. Any such DC level will then lead to a change in the average level at the output of the chopper, and so to an error in the detected phase difference.
US patent application 2002/027459 discloses a method and system for producing frequency multiplication/division by any non-integer output signal frequency relative to a reference signal frequency of a PLL, while simultaneously maintaining low jitter.